In conventional automotive braking systems, a master cylinder converts driver exerted brake pedal force into a corresponding hydraulic pressure, which is proportioned among the front and rear brakes. In power assisted braking systems, a vacuum booster is interposed between the pedal and the master cylinder to amplify the force applied to the master cylinder by using engine vacuum to create a pressure differential across one or more diaphragms coupled to the master cylinder.
As an alternative to the above-described conventional braking system, it has been proposed to use electrically powered boost units to develop the amplified brake pressures. Such systems--referred to herein as electro-hydraulic, or EH, systems--can advantageously be used, for example, in electric vehicles where there is no convenient vacuum source. Even in vehicles powered by an internal combustion engine, EH braking systems can be used to advantage for integrating advanced braking controls such as anti-lock braking, traction control and collision avoidance.
One example of an EH braking system is disclosed in the U.S. Pat. No. 5,246,283 to Shaw et al., issued Sep. 21, 1993, and assigned to the assignee of the present invention. In that system, the master cylinder pressure is coupled through normally open solenoid operated fluid valves and electrically powered boost units to the individual service brakes. In normal braking, the solenoid operated valves are activated to isolate the master cylinder from the service brakes, and the electrically powered boost units are activated to develop brake pressures based on a desired brake effort determined by an electronic controller. In the event of an electrical failure, the solenoid operated valves return to their normally open state, re-coupling the master cylinder to the service brakes, allowing continued braking with the manually developed master cylinder pressure. A system of this type is referred to herein as a manual back-up EH system.
A drawback of EH braking systems in general is that electric power consumption by the boost units can be considerable during extended idling when a high level of braking force is not actually required. This can occur, for example, while waiting for a traffic light, particularly if the driver is exerting significant brake pedal force. In addition to the unnecessary power consumption, this condition causes unnecessary heat generation in the boost units and controller, possibly adversely affect their durability.